1. Field of the Invention
This invention relates to ballasts for fluorescent lamps and, in particular, to a low cost apparatus for addition to conventional fluorescent fixtures for selectively increasing or decreasing light output.
2. Brief Description of the Prior Art
It has been well known for many years that high frequency current will operate a fluorescent lamp more efficiently, i.e. more light will be produced for the same amount of energy. In recent years, solid state ballasts have been developed that drive the lamps at high frequency and provide a significant energy saving. U.S. Pat. Nos. 4,277,728, 4,563,616 and 4,684,850 describe systems for adjusting the light level in order to maintain adequate light while using a minimum of energy.
In general, one can adjust light output by controlling either the frequency or power to the lamp. In the prior art, power to the lamp is varied via pulse width modulation, of either the line frequency or a high frequency obtained from a converter-inverter system.
It is also known to provide what can be considered a quasi-high frequency by chopping the voltage from the power line, e.g. U.S. Pat. No. 3,619,716. In this patent, the chopper is described as in series with lamp and its heaters.
In U.S. Pat. No. 3,913,002, an NPN transistor is in series with the ballast inductor, the lamp, and a current sensor. The line current per se is not chopped. Instead, a rectified, filtered current is chopped.
In U.S. Pat. No. 4,353,711, the chopper circuit is disclosed as connected to the DC terminals of a diode bridge, while the AC terminals of the bridge connected in series with the lamp. As with the previously noted patents, the heaters are subjected to the chopped current, which often causes problems during dimming.
It is also known to provide a bidirectional switch by connecting a transistor across the DC diagonal of a diode bridge, e.g. as in U.S. Pat. No. 4,375,608 and in the '002 patent already noted.
Further, it is known to use a photodiode in the negative feedback path of an inverter type of ballast; U.S. Pat. No. 4,672,300 (FIG. 3).
Some of these systems require replacement of existing ballasts with the new system, a cost many customers are unwilling to bear. In view of this, customers have turned to other means to save energy in applications where lighting reductions could be tolerated. These have involved removing some of the lamps from the fixtures, often resulting in uneven lighting conditions, turning off some lamps, relying instead on daylight, or installing small devices called "light reducers", between the ballast and lamp. Light reducers lower the lamp luminance by a predetermined, fixed amount.
This method of fixed light reduction is accomplished by the use of a capacitive reactance between the lamp and ballast, which has been found to have an adverse effect on some ballasts, causing excess heating or unpredictable performance. In some cases the wave shape of the current flowing through the lamp (crest factor) is altered enough to shorten lamp life. Crest factor is the ratio of the peak amplitude of the lamp current to the r.m.s. value of lamp current.
In view of the foregoing, it is therefore an object of the present invention to improve the efficiency of a conventionally ballasted lamp.
It is a further object of the present invention to provide a means for controlling the light output from a conventionally ballasted fluorescent lamp without subjecting the lamp heaters to reduced current.
In accordance with another aspect of the present invention, it is an object to enable the luminance of the lamp to be adjusted by either local or remote means.
In accordance with another aspect of the present invention, it is an object to make the fixture responsive to ambient light so that relatively constant illumination is available automatically.